1. Technical Field
The present invention relates to a line head control method, an image forming method, and an image forming apparatus capable of correcting exposure spot displacement to prevent deterioration of image quality.
2. Related Art
An exposure light source of an image forming apparatus which includes a line head using LED is known. JP-A-5-261970 proposes a technology which corrects displacement of exposure spot in a sub scanning direction of an LED line head containing light emission elements in zigzag shape. According to this technology, odd number data and even number data are separated from one another, and each of the data is stored at a write address shifted from the other data by the amount of the difference between odd number light emission element line and even number light emission element line when each data is written to an odd number frame memory and an even number frame memory. Then, exposure spot displacement between odd number dot and even number dot is corrected by a value integer number times larger than the exposure spot diameter (single dot diameter) through sequential read of data from the respective frame memories in synchronization with one strobe signal (in synchronization with line data cycle).
According to the method disclosed in JP-A-5-261970, in case of electronic picture printer using an intermediate transfer belt, for example, there is a possibility that exposure spot displacement cannot be appropriately corrected. This point is now explained with reference to FIGS. 19A and 19B. FIG. 19A shows a line head 10, a photosensitive body 41, an intermediate transfer belt 50 wounded around a drive roller 51 and a driven roller 52 (transfer roller) and rotating in an R direction indicated by an arrow, and recording sheet P fed in an S direction indicated by an arrow such that a toner image is transferred to the recording sheet P at the position of the transfer roller 52. In a typical electronic picture printer using the intermediate transfer belt, the tension of the intermediate transfer belt is controlled according to the ratio of the rotation speed of the photosensitive body 41 to the rotation speed of the intermediate transfer belt 50, i.e., the rotation speed of the drive roller 51 so as to prevent transverse stripes (banding) regularly generated when the toner image is transferred onto the recording sheet P.
In this step, the image expands and contracts in the sub scanning direction depending on the ratio of the rotation speed of the photosensitive body 41 to the rotation speed of the intermediate transfer belt 50, i.e., the rotation speed of the drive roller 51. Thus, the pitch between dots in the sub scanning direction of the image (exposure spot pitch) becomes a value other than a value integer number times larger than the exposure spot diameter (single dot diameter), that is, becomes a value non-integer number time larger than the exposure spot diameter in some cases. FIG. 19A shows the case of the photosensitive body 41 rotating at a low speed and the drive roller 51 rotating at a high speed. In this case, the intermediate transfer belt 50 is tensioned. FIG. 19B shows the case of the photosensitive body 41 rotating at a high speed and the drive roller 51 rotating at a low speed. In this case, the tension of the intermediate transfer belt 50 is reduced, and relaxation Rx of the tension is produced.
In this case, the exposure spot displacement is corrected only by a value integer number times larger than the exposure spot diameter (single dot diameter) according to the structure shown in JP-A-5-261970. Thus, accurate correction cannot be achieved when the exposure spot pitch in the sub scanning direction is a value non-integer times larger than the exposure spot diameter. For example, when a linear latent image is formed in the axial direction of the photosensitive body (main scanning direction), decimal times larger part of the non-integer times larger value cannot be corrected. In this case, a small gap is produced in the rotation direction of the photosensitive body (sub scanning direction). As a result, the image quality is deteriorated.